The invention relates to an automotive brake cylinder in which a pressure medium actuated brake piston is guided. The brake piston delimits a brake chamber and is connected to a brake piston rod acting on brake actuating elements, wherein a pressure medium connection for supplying pressure medium to and/or removing pressure medium from the brake chamber is provided. The brake piston rod together with the brake piston pivots into positions tilted from a center axis of the brake cylinder during an actuation stroke.
A brake cylinder of the above-mentioned type is disclosed, for example, in DE 197 56 519 A1. In this case, the constructional unit consisting of a brake piston and brake piston rod is coupled via the piston rod to a rotary lever of a brake application device of a disc brake of a commercial vehicle. The rotary lever is mounted at the other end on a brake caliper of the disc brake via a brake application shaft. The brake application shaft has a cam contour which, upon rotation about the longitudinal axis thereof, results in a relative displacement of the brake caliper and of a brake pad in the direction of the brake disc. However, upon a piston stroke of the brake piston in the brake cylinder, the coupling point of the brake piston rod on the rotary lever rotates about the brake application shaft, resulting in tilting or pivoting of the constructional unit consisting of the brake piston and brake piston rod within the brake cylinder.
In order to permit pivoting movements of the brake piston in relation to the brake cylinder, the brake piston is in the form of a flexible piston diaphragm, which is fastened via the radially outer circumferential border thereof to the brake cylinder and is supported by a brake piston rod plate connected to the brake piston rod. Owing to the elasticity thereof, a piston diaphragm of this type is capable of compensating for pivoting movements of the brake piston rod occurring during a piston stroke.
However, as the stroke becomes larger, a piston diaphragm of this type is placed partially against the radially inner cylinder wall of the brake cylinder, as a result of which the piston diaphragm surface, which is effective with regard to the pressure of the pressure medium, changes in relation to the piston stroke. This necessitates a deviation in the desired linearity between the piston stroke and brake force generated. Furthermore, a piston diaphragm of this type requires a relatively large construction space in the radial direction because the border thereof has to be fastened or clamped to the radially inner cylinder wall of the brake cylinder.
It would therefore appear appropriate, by contrast, to seek a solution in which the brake piston is of rigid design, not deformed when pressurized and is guided with the radially outer circumferential border thereof directly on the radially inner cylinder wall of the brake cylinder. However, when a rigid brake piston is used instead of a flexible piston diaphragm, there is the problem that the small air gap present between the radially outer circumferential surface of the brake piston and the radially inner cylinder wall of the brake cylinder should be the same size in each pivoted position of the brake piston in order to ensure consistent guidance of the brake piston in the brake cylinder in each pivoted position of the brake piston.
The invention is therefore based on the object of developing a brake cylinder of the above-mentioned type such that the disadvantages described above are avoided.
This and other objects are achieved by an automotive brake cylinder in which a pressure medium actuated brake piston is guided. The brake piston delimits a brake chamber and is connected to a brake piston rod acting on brake actuating elements, wherein a pressure medium connection for supplying pressure medium to and/or removing pressure medium from the brake chamber is provided. The brake piston rod together with the brake piston pivots into positions tilted from a center axis of the brake cylinder during an actuation stroke. The brake piston is of rigid design and is provided on the radially outer circumferential surface thereof with a guide surface which is guided along a radial inner cylinder wall of the brake cylinder and is in the form of a spherical segment of an imaginary sphere. The center point of the imaginary sphere lies on the center axis of the brake cylinder. At least one elastic sealing element, which is movable radially in a radially outer annular groove of the brake piston on the side thereof which faces away from the radially inner cylinder wall of the brake cylinder, delimits, together with a groove base of the annular groove, an annular space which is fluidically connected to the brake chamber such that the contact pressure of the outer diameter of the sealing element against the radially inner cylinder wall of the brake cylinder depends on the pressure prevailing in the brake chamber.
According to a first aspect of the invention, the brake piston is of rigid design, i.e. it is only negligibly deformed, if at all, under the changes in pressure or pressurizations occurring during operation. Furthermore, in contrast to a piston diaphragm, the brake piston together with seals and guide elements is guided and displaced uniformly within the radially inner cylinder wall of the brake cylinder, i.e. all of the components of the brake piston execute the same piston stroke at the same time.
As a result, the brake cylinder can be constructed to be smaller in diameter because, in contrast to a piston diaphragm, no border fastening is necessary, and the brake piston is guided coaxially within the radially inner cylinder wall of the brake cylinder. Therefore, with the same diameter, a cylinder-piston drive having a rigid brake piston can produce a greater force than one with a flexible piston diaphragm. Therefore, “rigid” is intended to be understood as meaning any material behavior making it possible to act upon the brake piston with operating pressure without the brake piston losing its function or leakages occurring by, for example, the radially outer piston border slipping out of contact with the radially inner cylinder wall of the brake cylinder. Furthermore, the desired linear characteristic of the piston stroke/brake force can be implemented because a rigid brake piston surface, which is effective in relation to the pressure medium, is always the same size irrespective of the piston stroke.
According to a further aspect, the brake piston is provided on the radially outer circumferential surface thereof with a guide surface which is guided along a radial inner cylinder wall of the brake cylinder and is in the form of a spherical segment of an imaginary sphere, the center point of which lies on the center axis of the brake cylinder. The plane of this guide surface, which is in the form of a spherical segment, is therefore arranged perpendicularly to the brake piston rod. The brake piston can therefore pivot out in relation to the center position thereof without the size of the air gap between the guide surface thereof and the radial inner cylinder wall of the brake cylinder changing. The guidance of the brake piston in the brake cylinder is therefore independent of the particular piston stroke and in particular independent of the pivoted positions of the brake piston.
According to a further aspect, the invention proposes at least one elastic sealing element which is movable freely radially in a radially outer annular groove of the brake piston and, on the side thereof which faces away from the radially inner cylinder wall of the brake cylinder, delimits, together with a groove base of the annular groove, an annular space which is fluidically connected to the brake chamber in such a manner that the contact pressure of the outer diameter of the sealing element against the radially inner cylinder wall of the brake cylinder depends on the pressure prevailing in the annular space. A substantial advantage of a sealing element of this type is that it has a self-reinforcing sealing effect.
Without the application of a pressure generated by pressure means, the sealing element bears under a slight internal stress and with correspondingly little friction against the radial inner cylinder wall of the brake cylinder. Therefore, the sealing action which is then not necessary is low, and the brake piston can be displaced under a slight friction in relation to the radially inner cylinder wall of the brake piston.
Upon application of a brake pressure in the brake chamber, the brake pressure passes into the annular space between the groove base and sealing element, as a result of which the latter is pushed with greater strength in the radial direction against the radially inner cylinder wall of the brake cylinder, thus increasing the sealing action. When the brake chamber is acted upon with pressure medium, the contact pressure of the sealing element in relation to the radially inner cylinder wall of the brake cylinder is consequently increased, this bringing about the then necessary greater sealing action in a self-reinforcing manner.
The combination of the features of the rigid brake piston with the tiltable mounting of the brake cylinder via a guide surface in the shape of a spherical segment and with a self-reinforcing elastic seal therefore affords the abovementioned advantages.
In a further development, the guide surface is particularly preferably formed on a guide ring held on the radially outer circumferential surface of the brake piston. A guide ring of this type can easily be exchanged, for example due to wear.
At least one sealing element of this type is preferably arranged offset axially with respect to a plane containing the center point of the imaginary sphere perpendicular to the center axis of the brake cylinder. There is then a lever arm between the sealing element and the center point of the imaginary sphere, about which the brake piston pivots together with the brake piston rod such that the eccentric arrangement of the sealing element with respect to the center point of the imaginary sphere has the effect that, upon pivoting of the brake piston about the center point, the contact pressure of the sealing element against the radially inner cylinder wall of the brake piston is higher in one circumferential region and lower in another circumferential region. This would result in a smaller sealing action in the circumferential region of the sealing element having the lower contact pressure per se. However, the lower contact pressure of the sealing element in the one circumferential region is compensated for by the annular space of the sealing element being acted upon with pressure medium from the brake chamber and therefore with rising contact pressure such that there is a favorable sealing action even in this circumferential region of the sealing element.
The smaller the axial offset is between the center point of the imaginary sphere, which forms the spherical segment as a guide surface, and the sealing element, the more uniformly the contact pressure between the sealing element and the radially inner cylinder wall of the brake cylinder is distributed with respect to the circumferential direction and the better is the sealing action. In order to obtain as small as possible an axial offset or lever arm between the center point of the imaginary sphere as the pivot point of the brake piston and the sealing element, the guide ring is therefore preferably arranged in the immediate vicinity of the sealing element.
Since the sealing element is designed as a sealing ring which has a fluidic connection between the annular space and a surface section of the head surface, which surface section lies with respect to a first sealing edge on the side which is pressurized to a greater extent, the pressure medium located under the foot part of the sealing ring can pass via the fluidic connection to the low pressure side, this causing a pressure loss on the high pressure side, which can be unambiguously detected by the tightness test which is to be carried out before the sealing arrangement is put into operation. This measure, consequently reliably prevents the sealing arrangement from being put into operation with the sealing ring fitted wrongly.
According to a particularly preferred embodiment, the fluidic connection is formed by at least one radially extending groove in that side wall of the sealing ring which is pressurized to a greater extent, i.e. in the side wall facing the brake chamber. A groove of this type can be produced by an only slight change in the injection mold for the sealing ring by means of the additional provision of a radially encircling projection, and therefore the measure can be implemented relatively cost-effectively.
According to a development of this embodiment, one end of the radial groove opens close to the first sealing edge and the other end opens at the end of that ring part of the sealing ring which faces the side which is pressurized to a greater extent. This produces a flow path which, with regard to the flow resistance, is advantageously short, this favoring the dissipation of pressure on the high pressure side in the event of wrong fitting.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawings.